Oxygen diffusion in columnar TiAlSiN coatings investigated by electron microscopy

A columnar TiAlSiN coating rich in Ti, deposited by cathodic arc evaporation PVD technique was subjected to a heat-treatment for 1 h at 1000 °C in air. The coating, with an initial thickness of 2 μm had a coarse-grained columnar structure with a chemically graded nanomultilayering consisting of alternating Ti-rich and AlSi-rich layers. After the heat-treatment, transmission electron microscopy cross-sectional images revealed, that an oxide layer with a thickness of about 1.2 μm has grown on top of the coating. This oxide had a layered Al2O3/TiO2 structure, which was found to be the consequence of the high diffusion rate of Al at 1000 °C. The interface with the coating was very rough and the EDS analysis showed that it was depleted of Al, which diffused to the upper surface of the oxide. In this area features looking like intercolumnar cracks in the TiAlSiN coating structure were formed. They were identified by EDS mapping as SiOx phase having an amorphous structure, as shown by HRTEM images, and resulted from the segregation of Si at grain boundaries and its oxidation during annealing.